The invention relates to terminally functionalized diene polymers and copolymers having reduced hysteresis in the carbon black reinforced and cured state. More particularly, the invention relates to diene polymers and copolymers containing terminals derived from fused-ring polynuclear aromatic compounds which can be utilized to form elastomer compositions for tire treads having reduced rolling resistance and good traction.
In recent years, those active in the tire industry have greatly increased their emphasis on the development of tires having both reduced rolling resistance and good wet traction properties. As is well known, that portion of the tire which exerts the greatest influence on rolling resistance and traction is the tread or tread rubber portion. Low rolling resistance is desirable from a fuel consumption standpoint while good wet traction is desirable from a safety standpoint. However, as a general rule, these properties have been found to conflict with each other. Thus, a reduction in rolling resistance generally leads to an almost directionally proportional reduction in wet traction while an increase in wet traction generally leads to an almost directionally proportional increase in rolling resistance.
The prior art has proposed a number of approaches to the solution of this problem. Such approaches have generally involved modifying the properties of the elastomer or elastomer composition utilized to form the tire tread in order to achieve the best possible balance between rolling resistance and traction. The approaches involving modification of the elastomer have generally been based on improving the interaction between the elastomer and the carbon black used in compounding the elastomer to prepare the tire tread composition in order to improve the dispersion of the carbon black into the elastomer. This has the effect of reducing the hysteresis of the tire tread composition which in turn results in low rolling resistance.
One known approach to modifying the diene polymer or copolymer elastomer to reduce the hysteresis of elastomer compositions formed therefrom involves coupling the living diene polymer or copolymer chains with metal halides. Thus, U.S. Pat. Nos. 4,383,085 and 4,515,922 describe the coupling of living diene polymer or copolymer chains obtained by anionic polymerization using an organolithium initiator with metal halides such as tin halides, silicon halides and the like. These patents indicate that tire treads formed from rubber compositions containing the coupled polymers have reduced hysteresis along with reduced rolling resistance and improved wet skid resistance.
Another known approach to modifying the diene polymer or copolymer elastomer to reduce the hysteresis of elastomer compositions involves terminating the living diene polymer or copolymer chains with certain compounds containing functional groups which are reactive with the lithium terminals of the living polymer as illustrated by the following patents:
U.S. Pat. No. 4,835,209 discloses the termination of living diene polymer or copolymer chains with carbodiimides. The patent discloses that rubber compositions containing such polymers have excellent performance characteristics with respect to tensile strength, impact resistance, low heat-generating properties and wear resistance without impairing wet skid properties.
U.S. Pat. No. 4,816,520 relates to terminally functionalized polymers, including diene polymers and copolymers and a process for their preparation. The reference discloses that the terminally functionalized polymers are prepared from living polymers obtained by anionic polymerization of olefinically unsaturated monomers by first reacting the living polymers with capping reagents comprising various nitrogen compounds including substituted imines (Schiff bases) and diaziridines and then reacting the capped polymer with a terminating agent which contains halogen or acid anhydride groups. Capping reagents which are disclosed include among others a compound of the formula ##STR1## wherein R.sub.1 is H, alkyl, cycloalkyl or aryl and R.sub.2 and R.sub.3 are each alkyl, cycloalkyl or aryl. Terminating agents which are disclosed include halogen compounds such as chloromethylstyrenes, acryloyl chloride, methacryloyl chloride, epichlorohydrin, etc.
U.S. Pat. No. 4,935,471 discloses the termination of living diene polymer or copolymer chains with a capping agent selected from the group consisting of (a) halogenated nitriles having the structural formula X-A-C=N wherein X is a halogen atom and A is an alkylene group of 1 to 20 carbon atoms, (b) heterocyclic aromatic nitrogen-containing compounds, and (c) alkyl benzoates. The only halogenated nitrile compound specifically disclosed is chloroacetonitrile. This patent discloses that compositions containing such polymers have reduced hysteresis and that tire treads made from such compositions have lower rolling resistance and better traction characteristics.